The invention relates to a method of allocating radio channels between the base stations of a radio system, having a plurality of base stations, and apparatus for implementing the method.
In radio systems used for telecommunications, especially, but not exclusively, those available for use by the general public, such as mobile radio telephony systems, traffic demand varies according to the time of day, from day to day, and with the season of the year. This phenomena is a problem with which all those engaged in the arts of operating, installing and designing such systems are familiar. In the first multi-base station radio systems, channel allocation was implemented when the radio systems were initially commissioned. Prior to commissioning measurements relating to field strengths, ranges of coverage, interference characteristics etc. were carried out. Redistribution of radio channels was carried out when new base stations and/or new channels were introduced to the systems. Redistribution of channel allocation was also carried out if it was found that the traffic capacity of the systems was inadequate. The utility of such radio systems remained constant for long periods because changes to channel allocation required a significant amount of work to be performed on the systems.
In multi-base station systems of the type described above it was discovered, as traffic levels increased, that traffic demand was concentrated on certain geographic areas. Furthermore these geographic areas changed with time, so that it became desirable to redistribute available radio channels between base stations.
Initially, channel redistribution was carried out by mapping the traffic demand at different times of day. This information lead to the conclusion that channel distribution should be changed at certain times of the day. However, it was soon found that this solution was too clumsy to be functional. To overcome this problem those engaged in the field of radio communications attempted to set up, in a variety of ways, automatic systems for distributing radio channels, between base stations, in an optimal manner.
For a proper analysis of traffic demand knowledge of the following factors is required:
the geographic location of base stations;
frequencies of the available channels;
the transmitter powers utilised;
interactions of the channels with each other in different circumstances; etc.
A variety of methods have been used for determining the optimal redistribution of channels between base stations, for example linear programming with a number of subsidiary conditions, reflecting the variables referred to above. However, one difficulty is that the formulation of the problem is particularly complex, and requires a computer aided solution. Computers are indispensable to today""s flexible radio systems.
The prior art methods all have the disadvantage that the mathematical models which they employ become unmanageable with increasing numbers of parameters and conditions. The solution to this problem is of comparable difficulty to that of the well known xe2x80x9ctravelling salesmanxe2x80x9d problem.
Furthermore it is not feasible, in practice, to carry out field measurements every time the utilisation of radio channels is to be changed.
The present invention relates to a method of determining the channel allocation for multi-base station radio systems which can be used in cases where, because of the complexity of the problem, the earlier methods cannot be employed. An apparatus for implementing the method is also described. A radio network which incorporates the present invention includes means for collecting information needed to make decisions on channel allocation, together with a central control device which calculates the channel allocation, as between different base stations, and allocates channels accordingly. The calculation of the channel allocation is made by reference to an analogous physical system which behaves in an analogous manner to a multi-base station radio system, namely a heat bath containing interacting particles. One group of particles represents a given allocation of channels to a given base station. By minimising the total energy of the system, the optimal channel allocation between base stations is determined.
According to a first aspect of the present invention there is provided a method of distributing channels between base stations in a multi-base station radio system by constructing a mathematical model from which can be determined a channel distribution which has an acceptable level of inter-channel interference, and assigning channels to the base stations in accordance with the predictions of the mathematical model, wherein the mathematical model is based on a heat bath having a plurality of particles immersed therein, in which
each base station of the radio system corresponds to one particle,
each radio channel corresponds to a discrete state of said particle,
selected channels are represented by different states of a particle so that changes in said distribution of channels are represented by changes in the states of the particles,
interference between base station channels are represented by the interaction between the states of different particles,
a change in the interference range coverage area ratio (C/I) is represented by a change in the energy of the thermal bath,
the number of particles and states in the mathematical model represents the total number of base stations and channels in the radio system which corresponds to that necessary to meet a predetermined traffic demand with the maximum acceptable level of interference between channels,
a first condition of the radio system is specified by the number of particles and the interaction between them in the heat bath, a redistribution of the radio channels or a change in the number of channels is represented in the model by a change in the states and interactions of the particles, the channel allocation is established by setting an initial temperature for the heat bath at infinity and then reducing the temperature in steps so that the states of the particles change in accordance with a stochastic process, and the energy of the heat bath is changed in order to achieve an energy content which corresponds to an acceptable interaction between particles and acceptable level of interference between base stations.
According to another aspect of the present invention there is provided, for a multi-base station radio system having a plurality of base stations, each base station having one or more radio channels allocated to it, a method of distributing available radio channels between base stations so as to reduce interference between radio channels allocated to different base stations to an acceptable level wherein a distribution of radio channels between base stations is determined from a mathematical model based on a plurality of particles in a thermal bath, in which particle(s) at a given location in the thermal bath correspond to a given base station, and the particle(s) have a plurality of states, each occupied state corresponding to a radio channel, the occupancy of states determined by a thermal distribution, and the particles having interaction energies corresponding to interference between base stations, a desired allocation of radio channels being determined by setting the initial temperature of the thermal bath at infinity and then reducing the temperature of the thermal bath in steps so that a local minimum energy is attained by a stochastic process in which the interaction between particles corresponds to an acceptable level of interference between base stations and determining the radio channel allocation from the distribution of occupied states of the particle(s).
According to a further aspect of the present invention there is provided in a multi-base station radio system having a plurality of base stations, each base station having one or more radio channels allocated to it, apparatus for distributing available radio channels between base stations so as to reduce interference between radio channels allocated to different base stations to an acceptable level, includes a control means for sending commands to individual base stations determining the radio channels allocated to the base station, which control means is arranged so that it can receive data on traffic demand in the radio system and the availability and utilisation of radio channels, the control means including a computer on which is run a mathematical model of the radio system from which a distribution of radio channels between base stations is determined, the mathematical model based on a plurality of particles in a thermal bath, in which particle(s) at a given location in the thermal bath correspond to a given base station, and the particle(s) have a plurality of states, each occupied state corresponding to a radio channel, the occupancy of states determined by a thermal distribution, and the particles having interaction energies corresponding to interference between base stations, a desired allocation of radio channels being determined by setting the initial temperature of the thermal bath at infinity and then reducing the temperature of the thermal bath in steps so that a local minimum energy is attained by a stochastic process in which the interaction between particles corresponds to an acceptable level of interference between base stations and determining the radio channel allocation from the distribution of occupied states of the particle(s).
According to yet another aspect of the present invention there is provided a multi-base station radio system having a plurality of base stations, each base station having one or more radio channels allocated to it, including apparatus for distributing available radio channels between base stations so as to reduce interference between, radio channels allocated to different base stations to an acceptable level, which apparatus includes a control means for sending commands to individual base stations determining the radio channels allocated to the base station which control means is arranged so that it can receive data on traffic demand in the radio system and the availability and utilisation of radio channels, the control means including a computer on which is run a mathematical model of the radio system from which a distribution of radio channels between base stations is determined, the mathematical model based on a plurality of particles in a thermal bath, in which particle(s) at a given location in the thermal bath correspond to a given base station, and the particle(s) have a plurality of states, each occupied state corresponding to a radio channel, the occupancy of states determined by a thermal distribution, and the particles having interaction energies corresponding to interference between base stations, a desired allocation of radio channels being determined by setting the initial temperature of the thermal bath at infinity and then reducing the temperature of the thermal bath in steps so that a local minimum energy is attained by a stochastic process in which the interaction between particles corresponds to an acceptable level of interference between base stations and determining the radio channel allocation from the distribution of occupied states of the particle(s).